We are flying in to Shannon on a Sat. morning, headed to Dingle the same day. The question is whether to take the coast road N67 into Tralee, or the N21 Road through Abbeyfeale, then on to Tralee and Dingle. We'll have all day to meander and stop and tour and play. Any sights not to miss on our way? We will be new to "wrong-side driving", so is either road better than the other? Also, is the transition in Limerick to find N69 or N21 tricky or fairly easy? Also, any suggestions for a B&B in Glendalough or County Wicklow?
You'd be going out of your way to take the N67, since you'd have to go around north to get to the ferry across the Shannon, as opposed to driving around through Limerick and down. The roads themselves should be comparable, along the lines of our US highways (not interstates), and you'll go through a series of roundabouts, which are usually well-marked.
If you get in early enough in the day, you might want to consider going a bit north to see the Cliffs of Moher and then continue to Dingle. In that case, the N67 route makes more sense because you could cross the Shannon on the car ferry and go straight down to Dingle that way.
I've only driven the ferry route so I can't comment on the N21 route. However, according to viamichelin.com, the distance to Dingle via the ferry route is actually shorter than via Limerick. (It will, of course, be slower.) If the weather is clear when you get to Tralee, drive over Connor Pass enroute to Dingle. You many not get another chance to enjoy the view from the top.
I highly suggest Stirabout Lane B&B in Rathdrum, which is near Glendalough. (http://www.goireland.com/wicklow/stirabout-lane-bed-and-breakfast-accommodation-bed-and-breakfast-id39701.htm#Photo-Gallery). It was our favorite B&B the entire trip. Rathdrum is a small town, but we found some nice pubs and a great pub for dinner.
I would think the N21 route would be the easier managed way to go as it is pretty much a straight shot. The only challenge will be to get through the roundabouts in Limerick while still getting your feet wet.
I stopped for a moment and thought about your question and thought it would be a bit much to go north only to go south to wait to take the ferry and then pay for the ferry.
You had me a little confused and lost until I finally found N67. If you're not going to the Cliffs, I don't see any reason to use N67, unless you just want to do a coastal drive, but you'll have better coast drives on Dingle Peninsula.
If you're not interested in the Cliffs, I'd suggest you go north on N18 from the airport towards Ennis, then head SW on R473 & R468 to the ferry. It's a nice easy drive along the Shannon River and would eliminate encountering the roundabouts at Limerick as you are learning to drive on the left. From Tarbert then you have another easy drive to Tralee and Dingle.
Sorry, but I have no suggestions for Glendalough or Wicklow areas. We have never stayed anywhere close to either.